Most of the power used by modern hard disk drives is consumed by the spindle motor. When the hard disk is initially started up, the motor may draw a peak level of power that is more than two times what it takes to keep the disk spinning. While in most cases even the peak start-up power usage is not substantial, there may be an issue when using multiple hard disks that attempt to spin-up simultaneously. Such an occurrence requires a sufficient power supply to withstand this initial demand.
As a solution to the above-described problem, staggered spin-up is implemented in systems where the host system may spin up the disk drives sequentially. Staggered spin-up significantly lowers design requirements and the cost of the power supply, and avoids overloading of the power supply, reducing the risk of damage to the power supply and the disk drives.
However, in a traditional host bus adapter (HBA), most of the physical layer (phy) reset sequence state machines are implemented in firmware, leaving staggered spinup a firmware task. The disadvantage of enabling firmware handle phy reset sequence and spin-up is that it adds real time handling requirement to the host CPU, thus slowing down the performance. Further, since host processors are moving further away from the control unit, putting more and more pressure on offloading part or all of the reset sequence state machines in hardware, making firmware implementation of staggered spin-up will become undesirable.